Inverse modeling of SO2 and NOx emissions over China using multisensor satellite data - Part 1: Formulation and sensitivity analysis

SO₂ and NO₂ observations from the Ozone Mapping and Profiler Suite (OMPS) sensor are used for the first time in conjunction with the GEOS-Chem adjoint model to optimize both SO₂ and NO_x emission estimates over China for October 2013. Separate and joint (simultaneous) optimizations of SO₂ and NO₂ emissions are both conducted and compared. Posterior emissions, compared to the prior, yield improvements in simulating columnar SO₂ and NO₂, in comparison to measurements from the Ozone Monitoring Instrument (OMI) and OMPS. The posterior SO₂ and NO_x emissions from separate inversions are 748 Gg S and 672 Gg N, which are 36 % and 6 % smaller than prior MIX emissions (valid for 2010), respectively. In spite of the large reduction of SO₂ emissions over the North China Plain, the simulated sulfate-nitrate-ammonium aerosol optical depth (AOD) only decrease slightly, which can be attributed to (a) nitrate rather than sulfate as the dominant contributor to AOD and (b) replacement of ammonium sulfate with ammonium nitrate as SO₂ emissions are reduced. For joint inversions, both data quality control and the weight given to SO₂ relative to NO₂ observations can affect the spatial distributions of the posterior emissions. When the latter is properly balanced, the posterior emissions from assimilating OMPS SO₂ and NO₂ jointly yield a difference of -3 % to 15 % with respect to the separate assimilations for total anthropogenic SO₂ emissions and ±2 % for total anthropogenic NO_x emissions; but the differences can be up to 100 % for SO₂ and 40 % for NO₂ in some grid cells. Improvements on SO₂ and NO₂ simulations from the joint inversions are overall consistent with those from separate inversions. Moreover, the joint assimilations save ∼ 50 % of the computational time compared to assimilating SO₂ and NO₂ separately in a sequential manner of computation. The sensitivity analysis shows that a perturbation of NH₃ to 50 % (20 %) of the prior emission inventory can (a) have a negligible impact on the separate SO₂ inversion but can lead to a decrease in posterior SO₂ emissions over China by -2.4 % (-7.0 %) in total and up to -9.0 % (-27.7 %) in some grid cells in the joint inversion with NO₂ and (b) yield posterior NO_x emission decreases over China by -0.7 % (-2.8 %) for the separate NO₂ inversion and by -2.7 % (-5.3 %) in total and up to -15.2 % (-29.4 %) in some grid cells for the joint inversion. The large reduction of SO₂ between 2010 and 2013, however, only leads to ∼ 10 % decrease in AOD regionally; reducing surface aerosol concentration requires the reduction of emissions of NH₃ as well.